Comment # 5 on bug 1174581 from Marius Kittler

I've also fixed this at some point within my projects. The tricky part is that
the clarifying sentence within the documentation is within the section for
`CMAKE_INSTALL_FULL_<dir>` (and not `CMAKE_INSTALL_<dir>`):

> The absolute path generated from the corresponding CMAKE_INSTALL_<dir> value. If the value is not already an absolute path, an absolute path is constructed ���

The "already" clarifies the possibility for `CMAKE_INSTALL_<dir>` to be
absolute. So any distro using absolute paths here is definitely correct.

Changing the macro to use relative paths can still be seen as a useful
workaround to deal with certain projects.

(It would also make sense to make the upstream documentation easier to
understand, e.g. the section of `CMAKE_INSTALL_<dir>` should already explicitly
clarify that the path can be absolute.)